This invention relates to a flat balance spring for a horological balance comprising a wound strip shaped to ensure an approximately concentric development of the balance spring and almost zero force on the pivots and on the fixing point, during the rotation of less than 360° of its inner end relative to its outer end in both directions from its rest position. This invention also relates to a balance wheel/balance spring assembly.
The non-concentric development of a balance spring fitted to a horological balance during the oscillation of the balance wheel/balance spring assembly results in an eccentricity of the center of gravity of the balance spring which, depending on the position occupied by the watch, causes the movement to run slow or fast, that is to say it reduces or increases the natural frequency of the balance wheel/balance spring system. This eccentricity of the center of gravity of the balance spring also causes the pivots of the balance to exert sideways pressure on the bearings.
These effects of imbalance of the balance spring and sideways pressures of the pivots destroy the necessary conditions of isochronism of the oscillations of the balance. Since the middle of the 18th century, watchmakers have been aware that the non-concentric development of the balance spring has a bad influence on isochronism and in particular that the sideways pressure caused by an eccentric balance spring on the balance pivots disturbs the rate and causes pivot wear. These same watchmakers therefore recommended forming one or two end curves, initially on cylindrical balance springs and, later, on an Archimedean type balance spring contained in a plane, which is known as the Breguet balance spring from the name of its inventor.
These curves were produced more or less empirically and corrected according to the results of the rate of the oscillator, until certain shapes rose to preference in the light of these results. It was several decades before the mathematics behind this end curve were studied by Edouard Phillips, thus supplying theoretical confirmation of the previous intuitions of watchmakers, namely that if the center of gravity of the balance spring is kept approximately on the balance staff as the balance wheel/balance spring system oscillates, the balance spring will exert relatively no sideways force on the pivots of the balance and its development will remain concentric.
The conditions described by Phillips are the same as those defined by the watchmakers who had deduced them themselves from their observations of the faults introduced by the balance spring, as compared with the rules governing the isochronism of an oscillating body described in the 17th century by Huygens.
The Breguet balance spring requires that an end curve be formed in a plane parallel to the plane of the flat balance spring. This requires the formation of two bends in opposite directions to form an inclined connecting segment between the balance spring and the parallel end curve.
A Breguet balance spring can be manufactured in various ferromagnetic or paramagnetic alloys, notably for self-compensating balance springs. However, it is much more difficult to manufacture it in a fragile material such as monocrystalline or polycrystalline silicon because the two reversed bends designed to allow formation of the Breguet end curve cannot be formed because a fragile material of this kind would break, and it is therefore necessary to resort to a technique enabling the formation of structures that are connected across a plurality of levels.
It has already been proposed that a technical effect comparable to that of the Breguet curve can be obtained on a flat balance spring by varying the thickness of the strip of the balance spring.
In U.S. Pat. No. 209,642 it is proposed that the thickness of the strip of the balance spring be increased gradually or discontinuously from the center to the outside of the balance spring.
CH 327 796 proposes modifying the cross section of the strip of the balance spring to make it stiffer, along an arc of not more than 180°, either in the center or on the outside. This modification is accomplished by bending, by addition of material (as by galvanic deposition or welding), or by thickness reduction (as by calendering or chemical etching).
U.S. Pat. No. 3,550,928 recommends stiffening the end curve of the balance spring with a non-rectangular cross section obtained by plastic deformation of part of the last turn.
EP 1 473 604 relates to a flat balance spring comprising on its outer turn a stiffened portion designed to make the deformations of the turns approximately concentric.
BE 526689 proposes varying a cross section of the strip of the balance spring along one or more parts of its length, or modifying the profile or adding to one or more parts of the strip a body (any body) designed to modify the flexibility of these parts. No further details are given as to these variations or modifications.
Emile and Gaston Michel, in their article Spiraux plats concentriques sans courbes [Concentric Flat Balance springs Without Curves], Bulletin Annuel de la Société Suisse de Chronométrie et du Laboratoire de Recherches Horologères, Vol. IV, 1957-1963, pages 162-169, Jan. 1, 1963, suggest giving part of the strip a v-shaped cross section. “This v-shaped part exhibits practically no deformation at high amplitudes. It now contributes nothing to the regulation and is as it were a dead part of the turn” (bottom of page 164 to top of page 165). This in effect neutralizes the balance spring for part of its length.
EP 1431844 relates to a balance spring whose cross section varies from one of its ends to the other. However, few details are given as to the form of variation of the cross section of the balance spring. The only information is that given in FIG. 11 and in the corresponding part of the description. The definition given on page 4, lines 55-57 speaks of “variable parallelepiped-shaped cross section”, “in this instance a rectangular cross section E toward the center which changes to become a square cross section E′ on the outside”. This definition, the only information given as to the type of variation, calls to mind a monotonic variation, because the two cross sections E-E′ between which the cross section varies appear to imply a continuous and monotonic variation of the cross section.
The question of the variation of the pitch illustrated in FIG. 10 of EP 1431844 is limited to a variation of the pitch along a radial axis F-F′ which gives to the balance spring an elliptical form. What this figure shows resembles rather a deformation of the balance spring spiral along one of the two axes than a variation of the pitch strictly speaking, and does not result in a functional balance spring, especially a balance spring whose turns do not touch each other in operation.
Lastly, in EP 1 593 004, the cross section of the strip of the balance spring decreases gradually from the center of the balance spring toward the outside.